1. Field of the Invention
This invention relates to a method for continuously carburizing low carbon coil stock and more particularly to a method for carburizing coil stock of less than about 20 mils thick wherein the carbon content is increased by rapid carbon diffusion.
2. Description of the Prior Art
It is well known in the art that the carbon content of steel can be increased by carburization. For example, U.S. Pat. No. 2,531,731 teaches the carburization of low carbon rimmed steel after cold reduction. Another method for carburizing steel is disclosed in U.S. Pat. No. 2,513,713. In this patent light gage, low carbon steel strip is heated to and maintained at an elevated temperature and continuously carburized by passing the strip through a sealed furnace in the presence of a carburizing atmosphere so that the atmosphere reacts with the strip. The strip is then quenched and normalized. The method disclosed in this patent, although teaching continuous carburization of steel strip, has some serious deficiencies. Namely, uniform carbon distribution across the width of the sheet is not obtained. This nonuniformity necessitates trimming the edges after carburization. To improve carbon distribution the patentee employs two forms of heating. The strip is heated by electric resistance heating which is referred to as internal heating and the carburizing chamber is also heated to avoid radiation heat loss from the strip. The resultant strip is thereafter quenched in a lead bath and then re-austenitized to provide "material soft enough to be handled without difficulty". From a metallurgical standpoint, it is reasonable to assume that the microstructure probably contains coarse pearlite, cementite and proeutectoid ferrite depending upon the final carbon content of the strip.
Although carbon distribution and strip microstructure significantly affect the mechanical properties of the strip these parameters are also important for another reason, namely, response to subsequent heat treatment. If, for example, the carburized strip is to exhibit a tensile strength in excess of 300 KSI the strip must receive a heat treatment so that an appropriate microstructure, such as fine grain tempered martensite, can be obtained. Such a microstructure cannot be practically achieved if the strip, prior to heat treatment, contains substantial amounts of coarse pearlite and proeutectoid ferrite and carbon distribution is not uniform. The product produced by the method disclosed in U.S. Pat. No. 2,513,713 contains a non-uniform carbon distribution and micro-constituents not amenable to a rapid response to heat treatment.
Carbon for diffusion into low carbon steel is supplied by enriching an endothermic carrier gas with a hydrocarbon gas. In the continuous carburization of steel strip the amount of hydrocarbon gas employed, viz, methane is generally maintained at about 5% by volume of the carrier gas. Controlling the amount of hydrocarbon gas added to enrich the carrier gas is important for two reasons, (a) an excessive amount of free carbon can be generated in the form of soot and can deposit on the surface of the carburized stock, (b) the amount of carbon available for carburization cannot exceed the amount that can be absorbed by diffusion into a low carbon stock of specific thickness.
It is common practice to supply only enough carbon that can be readily absorbed by the stock. This is accomplished in the continuous carburization of steel strip by maintaining a low percentage of hydrocarbon gas in the carburization gas. A result of keeping carbon availability low is long residence times within the carburizing furnace. To reduce residence time and carburizing costs carbon availability could be increased. To do so however would result in sooting on the surface of the carburized stock. Therefore, carbon availability, cross-sectional area and minimum soot formation must all be considered when considering any carburizing process. To achieve adequate carburization and no sooting the prior art employs long residence time and low carbon availability. As used hereinafter carbon availability is defined as the ratio of: pounds of carbon per hour entering the furnace to pounds of steel per hour passing through the furnace.
The method of the present invention rapidly carburizes steel strip by passing the strip through a furnace so that the residence time is of a short duration and thereafter treating the carburized strip in such a manner so as to prevent the formation of proeutectoid ferrite. The high through-put thus obtained permits in-line quenching, after carburization, thereby developing a unique microstructure.